Location:

Suggestions for Modification

With minor modifications this exercise could be adapted to most sites characterized by acid mine drainage.

Summary:

The field site is at the confluence of Deer Creek and the Snake River in Montezuma, Colorado, where acidic metal-rich waters contaminated by mining activity (Snake River) mixes with less acidic metal-poor waters of the pristine Deer Creek. This site is an excellent place to engage students in the process of science. Therefore the exercise focuses on the development and testing of a hypothesis. The exercise is intentionally open-ended so that it can be used with students with a wide range of geologic backgrounds.

Context:

This exercise was developed by participants of the PKAL Summer 2000 Institute, Earth and Planetary Science Workshop (July 18, 2000). This activity was a field trip titled "Field Based Undergraduate Research in an Introductory Environmental Science Course", lead by Dorothy Merritts of Franklin and Marshal College. The objective was to develop a field exercise that incorporates the learning cycle (exploration, knowledge construction, and application). The exercise is also designed to develop the student's basic skills such as map reading, drawing of sketches and or cross-sections, and collecting data. The author compiled the background information and modified the exercise after "field testing".

Goals:

This is an open inquiry activity with the focus on the "process of science" rather than the specific content. Students pose the problem, determine the method of investigation, collect and interpret the data.

Design:

Have the students orient themselves on the topographic map. They should locate some major topographic features (Satafe Peak, Sullivan Mountain, Teller Mountain) and determine which stream is the Snake River and which one is Deer Creek. Tell the students the objective of the exercise is to gain an understanding of the physical and geochemical hydrology of this system. They should make a sketch map of the area by walking up the Snake River, Deer Creek, and downstream from the confluence. Tell them they are expected to make observations and take notes, paying particular attention to the characteristics of the stream bed.

After approximately one hour reassemble the group and have them share their observations. Ask the students to make suggestions or pose some questions that could explain their observations. Divide the class into small groups and ask each group to formulate a hypothesis and determine how they will test their hypothesis. At this point students should be made aware of the equipment and tools available for them to collect data.

The next 1 to 2 hours should be spent making additional observations and collecting the data. At the base camp the groups will compile their data, summarize their findings and draw conclusions. Each group will share their results with the class. In the discussion that follows it should be determined if the results of the different groups are consistent with each other. If the results are inconsistent, how can they modify their hypothesis to address these inconsistencies and what additional data should be collected. If the results are consistent, how can they combine their results to get an overall picture of the system. Finally the instructor should share with the students the results of previous studies and environmental implications of the study.

Notes and Tips:

Geology, Hydrology and Geochemistry of the Site

The study area lies within the Montezuma District of the Colorado mineral belt. The Snake River drainage basin is underlain by the Idaho Springs Formation, composed of sillimanitic micaceous gneisses and schist. The Montezuma sheer zone crosses the Snake River drainage basin. The Swandyke Hornblende Gneiss underlies the Deer Creek drainage basin. Mineralization of the Precambrian host rocks resulted from hydrothermal fluids associated with the intrusion of the Tertiary Montezuma Stock, composed of aplite and porphyritic quartz monzonite. Mineralization was concentrated along the major faults and sheer zones. The hydrothermal vein fillings are predominantly pyrite, sphalerite, galena, quartz, Mn-carbonates, and barite.

The discharge of the Snake River and Deer Creek is similar at this site and their drainage areas are 11.7 and 10.4 Km2, respectively. The streams are covered with ice from December through early May, with a sudden increase in discharge following the spring snow melt. The streams discharge as much as 2 m3/s during May and June, but typically flow at 0.1 to 0.5 m3/s (Bencala and others, 1987).

Above the confluence the Snake River has a low pH (3.5-4.3) and high concentration of trace metals (Al 4.0 mg/L and Fe 0.7 mg/L). Deer Creek has a pH range of 6.5 to 8.0 and low concentrations of trace metals. Downstream from the confluence the pH ranges from 5.5 and 6.5 (McKnicht & Feder, 1984). DOC decreases 40% at the confluence, due to adsorption onto metal hydroxides. The aquatic community of Deer Creek is characterized by abundant diatoms and mayflies. Snake River above the confluence contains abundant liverwort, sparse diatoms and stoneflies. Below the confluence the aquatic community is very sparse. (McKnicht & Feder, 1984)

The bed of the Snake River above the confluence is covered with a reddish brown precipitate and the pebbles are highly weathered and easily fractured. In Dear Creek the pebbles are less weathered and coated with a black Mn-oxide. Below the confluence but before the culvert the bed of the stream was covered with a reddish brown precipitate. The amount of aluminum precipitate seems to increase after the water has flowed through the culvert under the road. Just downstream from the culvert the west side of the stream has the reddish brown precipitate and the east side has an off-white aluminum rich precipitate. Further downstream the off-white precipitate extends across the entire width of the stream. Aluminum precipitate appears to be more abundant in areas with increased turbulence. A "slimy" bacterial film was observed on some of the rocks coated with aluminum precipitate.

Directions to Site

From Keystone Colorado drive east on Rt 6 past the Snake River Saloon. Turn right onto Deer Creek Road.

From Denver Colorado take I-70 West to Rt 6 West (exit 216). Eight miles after Loveland Pass, turn left onto Deer Creek Road. The turn off is just after entering Keystone at the parking area for the River Run area. The road is also called Montezuma Road. Follow signs to Montezuma, approximately 8 miles, and continue through town 1.5 miles. There will be an abandoned mine spoil on the right and a large pull off where you can park. The study area is located in the Montezuma Colorado Quadrangle (USGS 7.5 Minute Series).

Downstream from the confluence showing aluminum and iron-rich precipitates on the Snake River side (right) of the stream bed. Also note the mine tailings in the background.

Equipment and Tools

The type and amount of equipment will vary depending on trip logistics and availability. Equipment marked with a * is essential.